1. Field of the Invention
The present invention relates to an image recording apparatus and an image recording method recording the respective ones of a plurality of unit images superposed with each other for forming a resultant image with a recording head comprising a plurality of recording sources.
2. Description of the Background Art
In a printing step, a resultant image may be formed by superposing a plurality of unit images such as a plurality of color-component images, for example, with each other. An image recording apparatus preparing printing plates corresponding to the respective ones of the plurality of unit images is employed in this printing step.
FIG. 15 shows an exemplary conventional image recording apparatus 100. In the image recording apparatus 100 shown in FIG. 15, a recording material 120 is mounted on the outer peripheral surface of a cylindrical recording drum 110, which is rotated in a main scanning direction (direction Y in FIG. 15) for scanning the recording material 120 with a recording head 130 in a subscanning direction (direction Y in FIG. 15) along an axis 111 of rotation of the recording drum 110. Thus, the image recording apparatus 100 two-dimensionally records unit images on a recording surface of the recording material 120 and prepares printing plates. In order to improve the recording speed, the image recording apparatus 100 employs the recording head 130 of a multichannel system capable of emitting a plurality of light beams B1, B2, . . . toward the recording material 120. The image recording apparatus 100 feeds the recording head 130 by a distance corresponding to the recording width of the plurality of light beams B1, B2, . . . in the subscanning direction during single rotation of the recording drum 110. Therefore, unit images are uninterruptedly recorded on the recording material 120.
If the plurality of light beams B1, B2, . . . are dispersed in quantity of light in this image recording apparatus 100, however, periodic fluctuation (nonuniformity) of densities results in the unit images recorded on the recording material 120.
It is assumed that the quantities of light of the plurality of light beams B1, B2, . . . are sinusoidally distributed as shown in FIG. 7, for example. In this case, the densities of the unit images recorded on the recording material 120 periodically fluctuate as shown in FIG. 8A. A period of this density fluctuation is equal to the recording width of the plurality of light beams B1, B2, . . . . FIG. 8A shows the density distribution of the unit images with reference to only one dimension in the subscanning direction.
When a resultant image is formed by superposing the plurality of unit images (images of printing plates of black, cyan, magenta, yellow, a special color etc., for example) recorded in the aforementioned manner with each other, periodic density fluctuation of the unit images is amplified by the superposition. Therefore, larger density fluctuation appears in the resultant image, as shown in FIG. 8B. Also when the quantities of light of the plurality of light beams B1, B1, B2, . . . are only slightly dispersed, it follows that large density fluctuation appears in the resultant image obtained by superposing the unit images recorded with the light beams B1, B1, B2, . . . with each other.
On the other hand, it is impossible in practice to completely uniformize the quantities of light of the plurality of light beams B1, B1, B2, . . . in the multichannel recording head 130. Therefore, the image recording apparatus 100 requires a technique capable of reducing the density fluctuation in the resultant image obtained by superposing the unit images recorded with the plurality of light beams B1, B1, B2, . . . dispersed in quantity of light.
The cause for the density fluctuation is not only the aforementioned dispersion between the quantities of light of the plurality light beams B1, B1, B2, . . . Fluctuation of the profiles of the individual light beams B1, B1, B2, . . . or dispersion of optical pulse widths modulated on the basis of recorded image signals may also cause density fluctuation.
This problem may be caused not only in an image recording apparatus preparing printing plates but also in an image recording apparatus recording the respective ones of a plurality of unit images superposed with each other for forming a resultant image with the same recording head. This problem may further be caused not only in an image recording apparatus recording unit images with light beams but also in an image recording apparatus recording unit images with a recording head comprising a plurality of independent recording sources.